Adjust type of ChemicalFormulaFeaturizer.powers.

[anoto-moniz]

The backend is opening up the type of powers to be floats. As such, this commit will deserialize powers as a list of floats. However, to preserve backwards compatability, the type of powers will be preserved, and a new field (powers_float) with the list as floats is introduced. In v4.0.0, the type of 'powers' will be changed, and 'powers_float' will be deprecated before its removal in v5.0.0.

PR Type:

• Breaking change (fix or feature that would cause existing functionality to change)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Maintenance (non-breaking change to assist developers)

Adherence to team decisions

• I have added tests for 100% coverage
• I have written Numpy-style docstrings for every method and class.
• I have communicated the downstream consequences of the PR to others.
• I have bumped the version in __version__.py

[kroenlein]

I think the migration strategy proposed is overkill given the actual impacts in Python of changing an int to a float. But I will disagree and commit.

Returning incorrect values when the object contains non-integer powers feels like mishandling data. That definitely violates minimum-surprise for me.

The approach suggested has been rejected in conversations via Slack.

[kroenlein]

A proposed workflow that alerts users when we cast their values without adding additional exceptions.

[StevenKauwe]

Catching up from OOO. These changes look good to me! 👍

They seem to preserve current client behavior while also adding the ability to work with floats if the user opts into it. Beyond that, looks like a lot already got discussed. This was helpful for catching up!

[kroenlein]

Given the small number of people who I think will be affected by this and the addition of a mutation warning, I'll approve.